[clang] [compiler-rt] [llvm] Skipping host target exports during cross-compilation (PR #209922)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-aarch64-libcxx` running on `linaro-flang-aarch64-libcxx` while building `clang,compiler-rt,llvm,runtimes` at step 16 "test-build-flang-rt-unified-tree-check-flang-rt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/89/builds/37120

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 16 (test-build-flang-rt-unified-tree-check-flang-rt) failure: test (failure)
...
0.552 [35/7/4] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/AccessTest.cpp.o
0.580 [35/6/5] Building CXX object flang-rt/unittests/Evaluate/CMakeFiles/reshape.test.dir/reshape.cpp.o
0.651 [35/5/6] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Allocatable.cpp.o
1.007 [35/4/7] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/Assign.cpp.o
1.026 [35/3/8] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/BufferTest.cpp.o
1.036 [34/3/9] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CharacterTest.cpp.o
1.117 [34/2/10] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/ArrayConstructor.cpp.o
1.210 [33/2/11] Linking CXX static library flang-rt/lib/Testing/libNonGTestTesting.a
2.252 [33/1/12] Building CXX object flang-rt/unittests/Runtime/CMakeFiles/RuntimeTests.dir/CommandTest.cpp.o
2.899 [32/1/13] Linking CXX executable flang-rt/unittests/Evaluate/reshape.test
FAILED: flang-rt/unittests/Evaluate/reshape.test 
: && /usr/local/bin/c++ -stdlib=libc++ -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -fdiagnostics-color -ffunction-sections -fdata-sections -Xclang -fno-pch-timestamp -O3 -DNDEBUG -stdlib=libc++ flang-rt/unittests/Evaluate/CMakeFiles/reshape.test.dir/reshape.cpp.o -o flang-rt/unittests/Evaluate/reshape.test  flang-rt/lib/Testing/libNonGTestTesting.a  lib/clang/24/lib/aarch64-unknown-linux-gnu/libflang_rt.runtime.a  -lLLVMSupport && :
/usr/bin/ld: cannot find -lLLVMSupport: No such file or directory
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/209922